Recently, optical discs, optical cards, optical tapes and the like have been developed as optical information recording media for allowing information to be optically recorded. Among these, optical discs have attracted attention as optical information recording media for allowing a large capacity of information to be recorded at high density.
One type of rewritable discs are phase-change optical discs. A recording layer used for a phase-change optical disc is reversibly changed to either an amorphous state or a crystalline state, depending on the conditions of heating by laser light and the cooling conditions. The recording layer has different optical constants when in an amorphous state from when in a crystalline state. Therefore, in the case of a phase-change disc, one of the two states is selectively formed in the recording layer in accordance with information to be recorded, and the resultant optical change (i.e., a change in transmittance or reflectance) is utilized. Thus, information recording and/or reproduction can be performed. In order to obtain the two states, information is recorded as follows.
A recording layer of an optical disc is irradiated with pulse-type laser light (referred to as a “recording pulse”) at a power for raising the temperature of the recording layer to equal to or higher than the melting point. As the laser light passes, the melted portion of the recording layer is rapidly cooled into a recording mark in an amorphous state. Alternatively, the recording layer is irradiated with focused laser light at a power for raising the temperature of the recording layer to a point equal to or higher than the crystallization point but equal to or lower than the melting point. Then, the portion of the recording layer irradiated with the laser light is placed into a crystalline state.
An optical disc is a recording medium which is exchangeably mounted with other optical discs on an optical disc recording and reproduction apparatus. Therefore, the optical disc recording and reproduction apparatus needs to stably perform recording on or reproduction from a plurality of optical discs. Even optical discs produced under the same conditions may have different recording mark states or may be differently influenced by thermal interference between recording marks, due to different thermal characteristics thereof which are caused by dispersions at the time of production and/or time-wise changes. This may result in different recording conditions including the recording power for the optical disc and the optimum edge position of the recording pulse.
In order to stably record information without being influenced by changes in such recording conditions, an optical disc recording and reproduction apparatus obtains recording conditions before recording information on an optical disc. Specifically, before recording information on an optical disc, an optical recording and reproduction apparatus performs test recording using a specific data pattern (referred to as “test information”), reproduces the test information, and measures the reproduced signal so as to obtain the recording condition. This process is referred to as “test recording”. Optical discs have an area used by the test recording, which is referred to as an “test recording area”.
A rewritable disc has a reproduction-only area pre-formed in a specific portion. The reproduction-only area includes convex and concave phase pits. In the reproduction-only area, information which does not need to be rewritten, for example, information on the optical disc itself or address information, is recorded. This area is also referred to as an “emboss area”.
An area in which information is recorded by forming a recording mark is referred to as a “recording and reproduction area”. In the recording and reproduction area, information which has a possibility of being rewritten is recorded.
A general rewritable optical disc has a data area in a central portion in a radial direction thereof, and has a lead-in area in a portion internal with respect to the data area, and a lead-out area in a portion external with respect to the data area. In general, an area for recording management information of the optical disc and/or a test recording area is provided in the lead-in area and the lead-out area.
Recently, there has been strong demand for the optical discs with a higher recording density. In response to this demand, a multi-layer recording medium having two or more information recording layers in a thickness direction of the disc is proposed. Information can be recorded on each of the information recording layers.
In such a multi-layer recording medium, each of the information recording layers often have different recording characteristics. This requires test recording to be performed for each information recording layer. One exemplary method of such test recording is disclosed in Japanese Laid-Open Publication No. 11-3550.
However, conventional methods do not consider the following phenomenon. When test recording is performed on an information recording layer of a multi-layer recording medium which is far from the laser incidence surface (the information recording layer far from the laser incidence surface will be referred to as a “second information recording layer”), the second information recording layer is influenced by the state of an information recording layer which is closer to the laser incidence surface (the information recording layer closer to the laser incidence surface will be referred to as a “first information recording layer”).
Laser light used for recording information on the second information recording layer may become non-uniform when the laser light has passed through a certain area of the first information recording layer. In this case, test recording would not provide accurate recording conditions.
The light transmittance of an information recording layer varies depending on whether information is recorded or not in the recording and reproduction area of the information recording layer. Therefore, the amount of laser light reaching the second information recording layer varies in accordance with the ratio of an unrecorded area and a recorded area in a light spot of the laser light on the first information recording layer, the light spot being formed before the laser light is used for test recording on the second information recording layer. As a result, accurate recording conditions are not obtained.
The light transmittance of an unrecorded area in the recording and reproduction area can be considered to be equal to the light transmittance of the reproduction-only area. However, the light transmittance of a recorded area in the recording and reproduction area is different from the light transmittance of the reproduction-only area. Accordingly, when test recording is performed on the second information recording layer, the amount of laser light reaching the second information recording layer varies also in accordance with the ratio of the reproduction-only area and the recording and reproduction area in a light spot on the first information recording layer (and the ratio of the recorded area and the unrecorded area in the recording and reproduction area).
The second information recording layer is influenced by the state of the first information recording layer when information (for example, user data information) is recorded in a recording and reproduction area of the second information recording layer, as well as at the time of test recording. In this case also, information cannot be accurately recorded, which reduces the signal quality used for reproducing the recorded information.
The present invention, to solve these problems of the prior art, has an objective of accurately obtaining optimum recording conditions for an optical information recording medium having two or more information recording layers, and another objective of accurately recording information on each of two or more information recording layers of an optical information recording medium.